1. Field of the Invention
The present invention relates generally to electric power converting devices and more particularly to an electric power converter for driving and controlling a load such as a variable-speed motor using a pulse width modulation (PWM) driving technique.
2. Description of the Related Art
In the recent past, power converting devices for variable-speed motor drive are becoming more widely used in the manufacture of a variety of electric equipment. A power converter typically includes an inverter circuit, which is packed in a one-chip integrated circuit (IC) device to meet the demand for reduction in size. The inverter includes in its output stage a plurality of pairs of power transistors for driving a variable-speed multiple-phase motor, typically a brushless direct current (DC) motor, in accordance with a pulse width modulation (PWM) drive technique.
For example, in the case of a three-phase brushless DC motor, three pairs of output transistors are arranged for electrically driving a plurality of winding coils to rotate a cone-shaped rotor of the DC motor. Usually, diodes are connected to the output transistors respectively. The three pairs of output transistors are connected in parallel with one another between a power supply voltage line and a ground potential line. Recently, insulated gate bipolar transistors (IGBTs) have been employed as the output transistors. One transistor of each pair, i.e., an IGBT coupled to the power supply line is called the "upper-stage transistor"; the other thereof is called the "lower-stage transistor" among those skilled in the art to which the invention pertains. In some cases, these upper- and lower-stage transistors may be called the "top-side transistor" and "bottom-side transistor" respectively.
The three parallel IGBT pairs are provided with driver circuits connected to the gate electrodes thereof respectively. These driver circuits are electrically fed by a common DC power supply unit. In particular, each of the three drivers for the upper-stage transistors is provided with a diode and a capacitor coupled thereto. The diode and the capacitor function to supply a corresponding driver with electric power. While the motor rotor is driven to rotate, when the lower-stage transistor of one output transistor pair turns on, current flows through the diode and the capacitor of a corresponding driver, thereby to charge the capacitor. The driver is thus activated by the charged capacitor. Activation of the driver causes the upper-stage transistor of the output transistor pair coupled thereto to turn on.
A controller is provided to be responsive to the output signals of three rotor-position detectors, which are arranged in the three-phase DC motor. The controller has control outputs connected through level shift circuits to the driver circuits for the upper-stage output transistors. The controller has other control outputs directly connected to the remaining three driver circuits for the lower-stage output transistors. The controller is provided with a circuit for providing a speed specify signal that is modulated in pulse width or pulse-width-modulated so as to represent a desired rotor speed of the DC motor.
In response to the rotor-position detection signals, the controller generates a plurality of control signals for pulse width modulation (PWM) drive of the three lower-stage output transistors. Conventionally, these PWM control signals are supplied to the driver circuits for the lower-stage transistors during mutually different time periods that are shifted by the electrical angle of 120 degrees and are equal in length to one another (120 degrees for each). Accordingly, each lower-stage output transistor repeats the turn-on and off switching operations in synchronism with the pulse width of the rotor-speed specify PWM signal during one 120-degree electrical angle period allocated thereto.
In each transistor pair, while its lower-stage transistor is PWM driven to repeat the turn-on/off switching operations, a corresponding upper-stage transistor coupled thereto is forced to turn off. When the upper-stage transistor in a certain pair turns on constantly during a specified 120-degree electrical angle period, the lower-stage transistor thereof is driven to turn off. At this time, one of those lower-stage transistors of the remaining two pairs is PWM driven to repeat the switching operations during the first half of the above 120-degree electrical angle period; the other lower-stage transistor of the remaining two pairs is PWM driven similarly during the second half of the 120-degree electrical angle period. By varying the pulse width modulation factor at these lower-stage transistors, the current supply to the motor may be done at a desired average value of current flow, thus controlling the motor so that its rotor rotates at a desired rate.
The significant problem of the conventional inverter is that a capacitor of large-capacity, which exhibits high charge storage capability, should be required as each of the capacitors coupled to the driver circuits for the lower-stage output transistors. The large capacitor is a serious bar to the miniaturization and cost-reduction of the inverter IC.
Such problem arises due to the following reasons. Looking at one of the three output transistor pairs for the purposes of explanation only, one power-feed capacitor for a certain driver circuit coupled to the one transistor pair is allowed to charge only during very shortened turn-on periods of the lower-stage transistor, which turns on and off repeatedly in synchronism with the PWM signal. When the lower-stage transistor turns on, the voltage at its drain electrode is substantially at the ground potential. Current rushes to flow from the DC power supply unit by way of the diode and capacitor associated with the driver circuit. This capacitor is then charged. The capacitor voltage rises in potential. The capacitor voltage is used to supply power to the driver circuit. The capacitor voltage is not given to this driver circuit for a certain electrical angle period (60-degree period, for example) before the upper-stage transistor will turn on again. The period will be as long as several hundreds of microseconds, if the rotation speed of DC motor is kept lower. To maintain the charged capacitor voltage for such a long period, the capacitor is required to have the enhanced charge storage capability, which is large enough to minimize the inherent natural discharge. Obviously, such capacitor is a massive element being larger in its physical volume.